Rheologically modified metal decorating and aqueous coating composition comprising copolymer latex and aminoplast

ABSTRACT

Disclosed are aqueous acrylic thermosetting resinous compositions having an alkaline pH and containing a specific rheological modifier in critical amount, and application to metal sheets, curing the same, and in a preferred embodiment shaping the metals to achieve metal decoration. By &#34;decorating&#34; is meant applying synthetic resinous coatings to the metal followed by a mechanical forming operation such as the formation of can bodies or screw-threaded bottle caps. The composition is thermosettable and comprises (A) a water-insoluble addition copolymer of certain monomers in latex form, (B) a water-soluble urea-formaldehyde or triazine-formaldehyde reaction product or a water-soluble methanol or ethanol ether thereof, or a combination of the aminoplast products, and, as an essential component, (C) a water-soluble polyethylene glycol having an average molecular weight of from about 400 to about 100,000. The copolymer contains small amounts of carboxyl groups, and amide or hydroxyl functionality, or both of the latter. The weight ratio on a solids basis of (A) to (B) is from 35:65 to 65:35, and the solids content is between 40 and 80 percent, the amount of said water-soluble rheology modifier being from 1 to 5 percent based on the total weight of (A) and (B), on a solids basis.

This is a continuation-in-part of application Ser. No. 333,594, filedFeb. 20, 1973, now abandoned.

This invention concerns metal decorating and thermosetting coatingcompositions which are applied from aqueous media to the surfaces to becoated. By "metal decorating" is meant coating a metal sheet with asuitable coating composition and then subjecting the sheet to variousmechanical forming processes. Examples of metal decorated items whichare in common use are beer cans, beverage cans, vegetable cans, screwcap jars and aerosol containers. The coatings for such items must beable to withstand a variety of fabrication treatments without crackingor chipping of the coating. These fabrication treatments range from verymild treatments (e.g., the formation of can bodies) to extremely severe(e.g., the fabrication of aerosol domes). Of course, the compositionscan also be applied to previously shaped articles such as mandrel-formedtwo piece can bodies, toothpaste tubes, etc.

In a typical process for the formation of a finished metal decorateditem, a pigmented base white coating composition is roller-coated onto ametallic substrate and the coated substrate is subjected to a bakingoperation. A decorative printing is then applied over the base whitecoat and, while the ink is still wet, a clear over-print varnish isroller-coated over the ink to protect it. The coated metallic sheet isthen subjected to a baking operation to develop final hardness. Thefinished coated metal sheets may then be stacked and stored for varioustime periods prior to fabricating into the final product. It isessential that the coating have good hot stack resistance to prevent themetal sheets from adhering one to another since this results in marringof the coatings. The coated sheets are finally fabricated into variousforms. The fabrication requirements will vary depending upon the end useof the material, e.g., whether it is to be fabricated into cans, lids,screw caps, etc. In addition to withstanding the various fabricatingoperations performed on the metal, many end uses require processing orsterilization operations which involve high temperature conditions. Insuch instances, the coating must possess good overbake properties.

In order to obtain maximum hot stacking properties for coatings, it isdesirable to use a thermosetting coating composition which results in ahigh degree of crosslinking when cured. However, the cured coating mustalso possess sufficient flexibility to permit the fabrication treatmentto which it will be subjected. If the coated metal is to be subjected tosevere fabricating operations, a "soft" thermosetting resin should beused; whereas if the coated metal is to be subjected to relatively mildfabricating conditions, a "hard" thermosetting resin may be employed.

Heretofore, metal decorating has involved the application of primerscomprising certain corrosion-protective pigments in non-aqueous vehiclesbased on a drying oil, such as linseed oil, a fast-drying varnish basecomprising natural resins, or a mixture of natural and synthetic resins,or an alkyd base modified with a urea-, melamine-, orphenol-formaldehyde resin. Such coating compositions are usuallysupplied in high-boiling solvents to insure good flow and leveling inthe thin films applied during a roller-coating operation. The solventsare frequently of an inflammable character and often are the type whichgive off noxious fumes during the coating operation. To cope with thefire and health hazards, protection is usually provided by way ofsolvent recovery systems. Moreover, recent interest in air pollutioncontrol has been aimed at reducing or eliminating organic solventemmission to the atmosphere.

There has long been a demand for thermosetting coating compositionswhich can be applied from an aqueous medium to avoid air pollution, firehazards and other problems which attend the use of organic solventcoating compositions. However, earlier aqueous coating systems, such asshown in U.S. Pat. Nos. 2,760,886 to Prentiss et al; 2,918,391 toHornibrook et al; and 3,033,811 to Brown et al have not proved fullysatisfactory for coating metals for uses according to the presentinvention by direct-roller coating machinery because they tend to dry onthe roller causing difficulties in clean up or blister when the wetproducts are put directly in an oven, or are not amenable topost-forming operations because of brittleness or to handling because ofsoftness. The greatest difficulty with water based systems is to obtainrheological properties suitable for direct-roll coating of the system.Thus, a composition is required which possesses flow and levelingproperties such that when applied by a direct-roll coater, it will forma uniform and smooth surface which does not contain striations. Thedeficiency of aqueous emulsions from the standpoint of flow and levelingis well recognized in the metal coating industry, see Paint and VarnishProduction, February, 1964, pages 28-33. The flow and leveling of thepigmented base coat is most critical since pigmentation noticeablyreduces flow in aqueous systems.

Application Ser. No. 130,808, filed Apr. 2, 1971, describes athermosetting coating composition which can be applied by direct-rollerprocesses to metals to provide films with good appearance which can bepost-formed and which provide, after baking, clear or pigmented coatingspossessing excellent water-resistance and solvent-resistance, excellentadhesion to a variety of substrates, and high gloss. The coatingcompositions described in that application are advantageously used inconnection with direct-roll coaters wherein the rollers are of lowDurometer hardness, e.g., about 12. However, since soft rubber rolls donot have good wear characteristics, it is desirable to provide athermosetting resin coating composition which can be applied by adirect-roll coater wherein the rolls are of high Durometer hardness,e.g., about 20-50, without sacrificing flow and leveling properties.This invention provides such a coating composition.

The coating compositions of this invention comprise an alkaline aqueousblend having a binder consisting essentially of:

A. a latex of a water-insoluble addition copolymer of (1) 28 to 70% of amonovinyl aromatic monomer, (2) 18 to 68% of at least one ester ofacrylic or methacrylic acid, (3) at least one olefinically unsaturatedmonomer having at least one of an amide and a hydroxyl group, and (4) anolefinically unsaturated monomer having a carboxyl group, the amount of(3) being from 1.5 to 10%, preferably 1.5 to 5% for metal decorating,when said monomer contains an amide group and from 1.5 to 15%,preferably 1.5 to 10% for metal decorating, when said monomer contains ahydroxyl group and the amount of (4) being from 0.5 to 5%, preferably0.5 to 2% for metal decorating, based on the total weight of monomers,the Tg of the polymer being below 45° C., and

B. a water-soluble condensation product of urea or a triazine withformaldehyde or a water-soluble methanol or ethanol ether thereof,

the weight ratio of A to B on a solids basis being from 35:65 to 65:35,the minimum film-forming temperature of the composition being no higherthan about 25° C., the solids content of the composition being between40 and 80% by weight, said composition containing from 1 to 5% by weightof a water-soluble rheology modifier having a number average molecularweight of from about 400 to about 100,000 comprising a polyethyleneglycol or a poly(ethylene oxide), based on the weight of A and B.

Desirably, B is the reaction product of a ureaformaldehyde adduct withmethanol, the mole ratio of urea: formaldehyde:methanol being in therange of 1/1.75-3/2-3.5; or a melamine-formaldehyde adduct withmethanol, the mole ratio of melamine:formaldehyde:methanol being in therange of 1/7-10/12-17, or a mixture thereof.

In copolymer A, (1) is preferably styrene or vinyl-toluene, (2) is anester of at least one of acrylic acid and an alkanol having from about 2to 12 carbon atoms or an ester of methacrylic acid and an alkanol havingfrom about 6 to about 14 carbon atoms, (3) is at least one ofmethacrylamide, acrylamide, hydroxyethyl acrylate or methacrylate orhydroxypropyl acrylate or methacrylate, and (4) is acrylic acid,methacrylic acid or itaconic acid.

If the coated metal is to be subjected to severe fabrication treatments,such as the fabrication of aerosol domes or bottle caps, then thecopolymer should be a "soft" copolymer. In this event, the weight ratiosof monovinyl aromatic/acrylic ester/amine-containing orhydroxyl-containing monomer or both/unsaturated acid is in the range of28-52/40-68/1.5-5/.5-2, preferably 35-50/45-60/1.5-3/.5-2, the totalbeing 100. The Tg of the "soft" copolymer should be below 0° C. and,preferably, below -15° C. The urea-formaldehyde adduct is usually neededfor severe post-coating fabrication operations, themelamine-formaldehyde adducts being well adapted to less severeapplication, or in cases where no deformation or mild deformation of thecoated product occurs.

For best results, the coating composition should contain a volatilebase, a tertiary amine being preferred, and should have a minimumfilm-forming temperature no higher than about 15° C.

When the coated sheet is to be subjected to mild fabrication treatmentssuch as formed into can bodies, or coated after fabrication, either a"soft" or "hard" copolymer may be used. Either of the aminoplasts, or acombination thereof, may be used for the less severe fabricationprocedures or for prefabricated articles.

The most preferred copolymer is a copolymer of styrene, butyl acrylate,at least one of hydroxyethyl methacrylate and methacrylamide, andmethacrylic acid.

One of the monomers utilized in a substantial proportion to prepare theaddition copolymer is a flexibilizing or "soft" monomer which may berepresented by the following formula: ##STR1## wherein R is H or alkylhaving 1 to 4 carbon atoms and R¹ is the straight chain or branchedchain radical of a primary or secondary alkanol, alkoxyalkanol oralkylthiaalkanol, the alkanol having from 2 to about 14 carbon atoms,the chain length depending upon the identity of R, examples being ethyl,methylpropyl, n-butyl, 2-ethylhexyl, heptyl, hexyl, octyl, propyl,2-methylbutyl, 1-methylbutyl, butoxybutyl, 2-methylpentyl,methoxymethyl, ethoxyethyl, cyclohexyl, n-hexyl, isobutyl,ethylthiaethyl, methylthiaethyl, ethylthiapropyl, n-octyl,6-methylnonyl, decyl, dodecyl, and the like. When R is alkyl and R¹ isalkyl, R¹ should have from about 6 to about 14 carbon atoms and when Ris H and R¹ is alkyl, R¹ should have from 2 to about 12 carbon atoms, inorder to qualify as a "soft" monomer.

Important properties of the copolymer are its toughness and flexibility,as well as its influence upon the minimum film-forming temperature (MFT)of the coating composition. The glass transition temperature (Tg) of thecopolymer and consequently the selection of monomers and proportionsthereof depends upon their influence on the Tg. "Tg" is a conventionalcriterion of polymer hardness and is described by Flory, "Principles ofPolymer Chemistry," pp. 56 and 57, (1953), Cornell University Press.While actual measurement of the Tg is preferred, it may be calculated asdescribed by Fox, Bull. Am. Physics Soc. 1, 3, p. 123 (1956). Examplesof the Tg of homopolymers and the inherent Tg thereof which permits suchcalculations are as follows:

    ______________________________________                                        Hompolymer of         Tg                                                      ______________________________________                                        n-octyl acrylate      -80° C.                                          n-decyl methacrylate  -60° C.                                          2-ethylhexyl acrylate -70° C.                                          octyl methacrylate    -20° C.                                          n-tetradecyl methacrylate                                                                           -9° C.                                           methyl acrylate       9° C.                                            n-tetradecyl acrylate 20° C.                                           methyl methacrylate   105° C.                                          acrylic acid          106° C.                                          ______________________________________                                    

These or other monomers are blended to give the desired Tg of thecopolymer. As is known, for a given number of carbon atoms in thealcohol moiety, the extent and type of branching markedly influences theTg, the straight chain products giving the lower Tg.

The coating composition of this invention similarly has a criticalmaximum MFT. MFT is determined by the method described in Resin Review,Vol. 16, No. 2 (1966). This is influenced not only by the Tg of theaddition copolymer, but by the plasticizers or coalescing agents usedand their amounts. At MFT values appreciably above this maximum,difficulties in obtaining a uniform coating film and lack of filmintegrity during deformation of the metal are encountered.

In addition to the flexibilizing monomer, the other essential monomersare the "toughening" or "hard" monomers, including the monovinylaromatic monomer, the unsaturated acid monomer, and the monomers havinghydroxyl and/or amide functionality. The hardness or softness of theacid and other functional monomers is not critical because of the smallamounts used. Styrene and vinyltoluene are examples of the monovinylaromatics.

The unsaturated carboxylic acid may be a simple monocarboxylic acid, ormay be a half ester or half amide of an α,β-unsaturated dicarboxylicacid, and salts thereof with a volatile base such as ammonia, or with avolatile water-soluble amine such as dimethylamine,dimethylethanolamine, triethylamine, triethanolamine, morpholine,N-methyl morpholine, picoline, and the like. Examples of copolymerizableethylenically unsaturated monocarboxylic or polycarboxylic acids aresorbic, cinnamic, vinyl furoic, α-chlorosorbic, p-vinylbenzoic, acrylic,methacrylic, maleic, fumaric, aconitic, atropic, crotonic, and itaconicacid, or mixtures thereof, with itaconic acid and the α,β-unsaturatedmonocarboxylic acids, particularly methacrylic acid and acrylic acid,being preferred. Other copolymerizable acid monomers include the alkylhalf esters or partial esters of unsaturated polycarboxylic acids suchas of itaconic acid, maleic acid, and furmaric acid, or the partialamides thereof. Preferred half esters are the lower alkyl (C₁ to C₆)esters such as methyl acid itaconate, butyl acid itaconate, methyl acidfumarate, butyl acid fumarate, methyl acid maleate, and butyl acidmaleate. Such partial esters and partial amides are considered to be"α,β-unsaturated monocarboxylic acids," and the term as used hereinincludes such esters and amides.

In addition to the acids, also present are an amide-containing monomersuch as acrylamide, methacrylamide, or the methylol or methoxymethylolderivatives thereof, or a hydroxyl-containing monomer such ashydroxyethyl or hydroxypropyl acrylate or methacrylate. Thehydroxyl-containing and amide-containing monomers may be used together,or singly.

Other ethylenically unsaturated copolymerizable monomers optionallypresent are useful in combinations with the above mentionedflexibilizing monomers and toughening monomers provided they do notadversely affect the desired properties of the copolymer (e.g., undulyraise the overall Tg). These may be represented by the formula; ##STR2##wherein R is as above. R² is preferably alkyl and is methyl or alkylhaving from about 13 to about 20 carbon atoms when R is H, and is alkylof from 1 to about 5 carbon atoms or alkyl of from about 15 to about 20carbon atoms when R is methyl. It can be seen from above that for alkylacrylates and alkyl methacrylates the Tg at first decreases with anincreased chain length of the alkyl group and then the Tg againincreases; i.e., both hard and soft monomers are known to occur in eachgroup of monomers. Examples of these hard monomers and other hardmonomers include: methyl acrylate, vinyl acetate, tetradecyl acrylate,pentadecyl acrylate, methyl methacrylate, ethyl methacrylate, t-butylacrylate, butyl methacrylate, and pentadecyl methacrylate.

The coating compositions described hereinbefore may be used as a cleartop coating or may be pigmented with a variety of pigments as set forthhereinafter.

The preferred emulsion copolymer, having a molecular weight of betweenabout 70,000 and 2,000,000, and preferably between about 250,000 and1,000,000 is made by the emulsion copolymerization of the severalmonomers in the proper proportions. Conventional emulsion polymerizationtechniques are described in U.S. Pat. Nos. 2,754,280 and 2,795,564.Thus, the monomers may be emulsified with an anionic, a cationic, or anonionic dispersing agent, about 0.05% to 10% thereof ordinarily beingused on the weight of the total monomers. The acid monomer and many ofthe other functional or polar monomers may be soluble in water so thatthe dispersing agent serves to emulsify the other monomer or monomers. Apolymerization initiator of the free-radical type, such as ammonium orpotassium persulfate, may be used alone or in conjunction with anaccelerator, such as potassium metabisulfite, or sodium thiosulfate.Organic peroxides, such as benzoyl peroxide and t-butyl hydroperoxideare also useful initiators. The initiator and accelerator, commonlyreferred to as catalysts, may be used in proportions of 0.01% to 10%each based on the weight of monomers to be copolymerized. The amount, asindicated above, may be adjusted to control the intrinsic viscosity ofthe polymer. The temperature may be from room temperature to 60° C. ormore as is conventional.

Suitable dispersing agents useful in emulsion polymerization includeanionic type such as the sodium salts of the higher fatty acid sulfates,such as that of lauryl alcohol, the higher fatty acid salts, such as theoleates or stearates or morpholine, 2-pyrrolidone, triethanolamine ormixed ethanolamines, or any of the nonionic types, such as ethyleneoxide modified alkyl phenols, of which tert-octyl phenol modified by 20to 40 ethylene oxide units is representative, ethylene oxide-modifiedhigher fatty alcohols such as lauryl alcohol, containing 20 to 50ethylene oxide units, similarly modified long-chain mercaptans, fattyacids, amines, or the like. Mixtures of nonionic and anionic dispersingagents are also useful. The preferred composition is prepared with anonionic emulsifier or such emulsifier is added after polymerization.

The amounts of emulsifier or emulsifiers required vary primarily withthe concentration of monomers to be handled and to a minor extent withchoice of emulsifier, monomers, and proportions of monomer. Generally,the amount of emulsifying agent is between 2% and 12% of the weight ofthe mixture of monomers and is preferably 4% to 7% of this weight. Ifthe dispersion is to contain a relatively low concentration ofinterpolymer somewhat more than the minimum emulsifying agent indicatedby the above rule may prove desirable. In such case, the concentrationof emulsifying agent in the aqueous solution may desirably be at least 1percent of this solution and may be as much as about 7 percent of theweight of the aqueous solution thereof.

As the addition polymerization catalyst, there may be used one or moreperoxides which are known to act as free-radical catalysts and whichhave solubility in aqueous solutions of the emulsifier. Highlyconvenient are the persulfates, including ammonium, sodium and potassiumpersulfates or hydrogen peroxide or the perborates or percarbonates.There may also be used organic peroxides, either alone or in addition toan inorganic peroxidic compound. Typical organic peroxides includebenzoyl peroxide, tert-butyl hydroperoxide, cumene peroxide, tetralinperoxide, acetyl peroxide, caproyl peroxide, tert-butyl perbenzoate,tert-butyl diperphthalate, methylethyl ketone peroxide, etc. Thepreferred organic peroxides have at least partial solubility in theaqueous medium containing the emulsifying agent. Choice of inorganic ororganic peroxidic catalyst depends in part upon the particularcombination of monomers to be interpolymerized, some of these respondingbetter to one type than the other.

The amount of peroxidic catalyst required is roughly proportional to theconcentration of the mixture of monomers. The usual range is 0.01% to 3%of catalyst with reference to the weight of the monomer mixture. Thepreferred range is from 0.05% to 0.5%, while the range of 0.1% to 0.25%is usually best. The optimum amount of catalyst is determined in largepart by the nature of the particular monomers selected, includingimpurities which accompany particular monomers.

Another suitable class of free-radical generating compounds are the azocatalysts. There may be used, for example, azodiisobutyronitrile,azodiisobutyramide, azobis-(α,α-dimethylvaleronitrile),azobis(α-methylbutyronitrile), dimethyl, diethyl, or dibutylazobis(methylvalerate). These and other similar azo compounds serve asfree-radical initiators. They contain an -N=N- group attached toaliphatic carbon atoms, at least one of which is tertiary. An amount of0.01% to 2% on the weight of monomer or monomers is usually sufficient.

In order to effect interpolymerization at a temperature below that atwhich coagulation might occur, it may be desirable to activate thecatalyst. This may best be accomplished by using a so-called redoxsystem in which a reducing agent is present in addition to the peroxidiccatalyst. Many examples of such systems are known. Agents such as asoluble sulfite, including hydrosulfites, sulfoxalates, thiosulfates,sulfites, and bisulfites can be used. Examples of these are sodiumhydrosulfite, sodium metabisulfite, potassium sulfite, zincformaldehyde-sulfoxalate, and calcium bisulfite. Tertiary amines mayalso be employed. Redox systems may be activated by the presence of asmall amount of polyvalent metal ions. Ferrous ions are commonly andeffectively thus used, a few parts per million being sufficient.

The amounts of reducing agent required vary somewhat with the choices ofperoxide initiator, reducing activator or agents, and metal promoter, ifused, also with the choice of emulsifying agent, and with the particularmonomers involved. Within the limits of about 0.5% to 6% with referenceto the weight of the mixture of monomers will be found the amount ofreducing agent for practically any system. The preferred amounts ofsulfite agent or equivalent fall in the range of 0.2% to 1%.

Copolymerization is best effected below about 80° C. A preferred rangeis 15° to 70° C., although slightly lower and somewhat highertemperatures are permissible. After most of the monomers have beenconverted to interpolymer, temperatures even higher than 80° C. may thenbe applied. In fact, after most of the monomers have interpolymerized,the resulting dispersion can be heated to boiling without breaking thedispersion. During interpolymerization, the temperature can becontrolled in part through the rate at which monomers are supplied andinterpolymerized and/or through applied cooling.

The polymerization process can be carried out batchwise, emulsifying theentire charge of monomers and proceeding with polymerization. It isusually advantageous, however, to start with part of the monomers whichare to be used and add more monomer or monomers as polymerizationproceeds. An advantage of gradual addition of monomers lies in reachinga high solids content with optimum control and with maximum uniformityof product. Additional catalyst or additional components of the redoxsystem may also be added as polymerization proceeds.

The water-soluble, heat-convertible condensation products of urea ortriazine (e.g., melamine) with formaldehyde and/or their derivativesobtained by reaction with ethanol or methanol can be prepared accordingto one of the following schemes: (1) control of reaction conditions sothat the degree of polymerization is kept very low, even to themonomeric stage, and (2) introduction of hydrophilic groups into themolecules of the polymeric condensates, Thus, they can be made bycareful control of reaction conditions as set forth in Schildknecht,"Polymer Processes," Vol. X, page 295 et seq. (Interscience Press,1956). The preparation of another class of compounds suitable in thepresent invention, such as N,N'-bis(methoxymethyl)urea is set forth inBull. Chem. Soc. Japan, Vol. XI, No. 3,239 (1936). In a preferredembodiment of this invention, a mixture of a urea-formaldehyde adductwith methanol and a melamine-formaldehyde adduct with methanol isemployed in the coating composition.

The coating composition of this invention preferably contains a materialdesignated as a "coalescent" or a "colsolvent." These materials aidfusion of the film during airdrying prior to baking and promote the flowof the coating composition during the baking cycle. However, becausethey are volatile they do not form a part of the finished coating. Theyare characterized by being low in water-solubility, good solvents forthe uncured polymer mixture, less volatile than water so that theyremain in the film after the water has evaporated, sufficiently volatileso that they are removed from the film before the end of the bakingcycle and not susceptible to hydrolysis in alkaline media either atambient or elevated temperatures. They also serve to lower the surfacetension of the aqueous system, making it easier to wet the metalsubstrate. Typical examples are isophorone(3,5,5-trimethylcyclohexene-2-one-1), 2-ethylhexanol, diacetone alcohol,dimethyl formamide, alkyl ethers of ethylene glycol and propyleneglycol, propanol, butanol and tributyl phosphate. The coalescent ispreferably present in the coating composition in an amount of from 10 to20% by weight based on the weight of the solids. The water solublecondensation product of urea or triazine with formaldehyde is usuallysupplied as an 80% solids solution in isopropanol or an equal volumemixture of isopropanol and butanol which are cosolvents. Therefore, thecoating composition will normally contain a coalescent even withoutadditional cosolvent being added. Any coalescent having the propertiesnoted above useful in latex paints is useful in the present invention.

The rheology modifiers which are used in the practice of this inventionare materials which are well known in the art. Polyethylene glycols aresold commercially under the name Carbowax. Their preparation isdescribed by Fordyce in J. Am. Chem. Soc., Vol. 61, pages 1905, 1910(1939). Preferably, a polyethylene glycol is used having a molecularweight of from about 400 to 20,000. Poly(ethylene oxide) is a polyetherobtained by polymerizing ethylene oxide. Such materials are sold underthe commercial designation Polyox. They may be prepared, for example, byprocesses as described in U.S. Pat. Nos. 3,365,404; 3,167,519;3,251,784; and 3,444,102. The poly(ethylene oxide) has an averagemolecular weight which does not exceed about 100,000. It may benecessary to use conventional methods to depolymerize products producedaccording to these patents to obtain this molecular weight or lowerones. Such depolymerized products are commercially available. Therheology modifier may be added to the coating system at any stage ofpreparation--i.e., it may be added to the thermosetting resin emulsionduring or after its preparation, it may be added to the composition atthe time of mixing with the condensation product of urea or triazinewith formaldehyde is added, etc.

Both the "Polyox" and "Carbowax" products are essentiallypolyoxyethylene polymers having linear chains and having terminalhydroxyl groups. The terminal groups can be varied to include estergroups, ether groups, epoxy groups, or other groups and herein wherever"polyethylene glycols" are referred to, the functionally equivalentlinear polyethers having terminal groups other than hydroxyls areintended to be included. Such terminal groups other than hydroxyl shouldhave no more than about two carbon atoms. With products having themolecular weights of the invention, the nature of the end groups on thelinear chain have essentially no effect upon the rheology-impartingproperties of the polymer. The "Carbowax" type of polyethylene glycolsare understood to be prepared by starting with water or ethylene glycoland ethylene oxide and polymerizing in the presence of an alkalinecatalyst. The "Polyox" polyethylene glycols are understood to beprepared by polymerization of ethylene oxide using a different catalystand in the absence of water or a starter such as a glycol. The latterproducts may have a molecular weight of several million, having arelatively broad molecular weight distribution as compared with thelower molecular weight polyethylene glycols prepared with a starter. Thewater-soluble polyethylene oxide materials useful in the invention maybe obtained by depolymerizing the products having molecular weights ofseveral million. As suggested above, the polyethylene glycols made witha starter generally have a very narrow molecular weight distribution.These facts show that the molecular weight distribution of the polyetheris not particularly critical, and where a molecular weight is given, itis to be understood that this is a number average molecular weight.

The exact mechanism by which the polyethylene glycols and poly(ethyleneoxides) act to modify the rheological properties of the coatingcomposition is not known. In the amounts used, they do not act asthickeners for the composition, but actually lower the viscosity of thecompositions. It is believed that these compounds may complex with theurea- or triazine-formaldehyde condensation product, thereby decreasingthe normal rapid increase in viscosity which occurs with a relativelysmall increase in solids encountered with the coating compositions inthe absence of of the rheology modifier. Thus, while the coatingcomposition without the rheology modifier may set up very fast resultingin striations in the final coating, the coating compositions containingthe rheology modifiers flow into a smooth coating before setting up. Bythe practice of this invention, there is obtained a synergistic effectbetween the urea or triazine-formaldehyde condensation product and therheology modifier to give improved flow. That is, the improvement inflow is considerably greater than can be attributed to the additiveeffect of the condensation product and the rheology modifier each byitself.

Since it is desired to have the mixture of the acrylic dispersion andthe aminoplast stable on prolonged storage, and since the aminoplast isreactive under acid conditions, the mixture must be made alkaline. Avolatile base, such as ammonia or a tertiary amine as discussed above inconnection with acidic monomer salts, is used to make the systemalkaline because tertiary amines will not react with the formaldehydeassociated with the aminoplast. The tertiary amines also function ascorrosion inhibitors when the coating compositions of the presentinvention are used for coating metal. The tertiary amine must besufficiently volatile that it will be driven from the film during thebaking operation. However, it must not be so volatile that it "flashes"from the film or gasifies if the coating composition is applied byspraying. Particularly preferred because of the balance of properties,availability and economy is triethylamine. The pH of the mixture shouldbe maintained in the range of 9 to 11 in order to ensure good storagestability. However, it is apparent that initial pH cntrol alone is notsufficient to insure adequate stability and retention of propertiessince samples neutralized to the desired pH range with ammonia haveinadequate stability on prolonged storage, although satisfactory filmsare obtained if the ammonia-neutralized compositions are used shortlyafter preparation. However, if the tertiary amine is the predominantnitrogen base present, small amounts of ammonia can be used withoutdeleterious effects. The amounts of amine used will vary depending onthe specific composition employed but will be in the range of 1 to 5parts by weight per 100 parts by weight of coating composition (solidsbasis). A preferred embodiment employs two parts by weight per 100 partsby weight of the coating composition (solids basis).

When pigments are employed, it is essential to employ a dispersant.While a wide variety of dispersants will satisfactorily dispersepigments, the effect of the dispersant on the properties of the finalfilm must be considered. Many dispersing agents remain in the final filmunchanged, thus seriously impairing the water-resistance of the film.Other dispersing agents will adversely affect the stabilities of thesystems into which they are incorporated.

A preferred embodiment employs as dispersants the ammonium and loweramine salts of polymeric carboxylic acids. Thus, the ammonium and loweramine salts of polyacrylic and polymethacrylic acids and similar saltsof the polymeric acid obtained by copolymerizing methyl vinyl ether withmaleic anhydride are suitable. A particularly preferred embodimentemploys the ammonium half amide salt or the diammonium salt of adiisobutylene-maleic anhydride copolymer having a number averagemolecular weight of from about 2,000 to about 4,000. The amount ofdispersant employed will vary depending on the amount and nature of thepigments used and the amount and nature of the composition employed asbinder. Generally, however, from about 0.3 to about 3.5 parts by weight(solids basis) per 100 parts by weight of pigment, will prove to beeffective for dispersing the pigment.

It appears that the dispersants of the type hereinbefore describeddecompose at the temperature employed in the baking cycle to liberateammonia or lower amine which is then volatilized. It is furtherpostulated that the carboxylic residuals react either with the amidegroup of the copolymer or with the aminoplast or both to becomeinsoluble. Regardless of the mechanism involved, the fact that it isobserved that this particular class of dispersants, when employed as setforth hereinbefore, do not detract from the excellent water-resistanceand other highly desirable properties of the films proves that suchcatalysis does occur. It has been proven that even clears which containa small amount of such dispersants exhibit better water- andsolvent-resistances than do the same compositions without dispersant,both samples being cured under exactly the same conditions. The amountof dispersant employed in clears varies depending on the amount ofaminoplast employed. From about 0.1 to about 1.0 part by weightdispersant per 100 parts by weight (solids basis) of the coatingcomposition will effect the desired catalysis.

The coating compositions of the present invention may be employed asclears, i.e., non-pigmented clear top coatings, or as pigmentedcoatings. If pigmented, the ratio of pigments to coating solids may bevaried widely, depending on the pigment employed and the specificapplication involved. Thus, the ratio of pigment to coating solids mayvary from 1 to 20 to 20 to 1. The clears are particularly useful as"over-coats," i.e., the so-called overprint coatings which are used toprotect decorative undercoats without detracting from the decorativeeffect. Because the clear coatings of the present invention exhibit goodclarity, high gloss, excellent solvent- and water-resistance, and highadhesion to a variety of surfaces, they are admirably suited for use asoverprint finishes.

Although the coating compositions of this invention are of particularutility for metal decorating, they can be applied to a variety ofsubstrates, the only restriction being the ability of the substrate towithstand the baking cycle which is essential in the processing of saidcoating compositions. Metals are particularly suitable, whetherprime-coated or umprimed. Thus, iron, steel, chrome-plated steel,tinplated steel, aluminum, copper, bronze, or brass surfaces,particularly in sheet or coil form with thicknesses of 0.05 to 0.20inches, prove to be excellant as substrates for the coating compositionsof the present invention. Ceramic surfaces and, in some instances, woodsurfaces, are also suitable as substrates. For roller coating such asreverse roll coating, the thickness is from 0.05 to 5 mils in thickness,preferably 0.2 to 1.5 mils in thickness when not cured.

A wide variety of pigments can be employed with the coating compositionsof the present invention. The pigments employed, however, must be stableand non-reactive under alkaline conditions, i.e., a pH from about 9 toabout 11. Typical pigments which are suitable include titanium dioxide,iron oxide, calcium carbonate, barytes and numerous types of clays.

The coating compositions of this invention are particularly suitable forapplication by a direct-roll coater although they may be applied byother means such as a reverseroll coater or a spray gun. A single rollcoater normally applies the coating to the substrate while theapplicator roll rotates in a pool of the coating composition. Thecoatings are then baked at a temperature of from about 250° F. to 350°F. for from about 1/2 to 10 minutes. The baking or curing operationvolatilizes all the volatile material in the film including anyremaining water, traces of monomer, coalescents, and the tertiary amine.It is particularly important that the tertiary amine be volatilizedsince it inhibits the cure of the aminoplast. The baking operationeffects the decomposition of the ammonium or amine salts of thepolymeric carboxylic acids, apparently releasing the acid from thecopolymer which may then react with the other components to becomeinsoluble. The baking operation causes the cure of the aminoplast whichcrosslinks and insolubilizes the entire film.

When applied to an Alodine aluminum (Q Panel 612, MIL-C-5441, 0.025 inchthick) dried, the dry film being 1 mil in thickness, and baked for 60seconds at 500° F., a "soft" resin coating of this invention endureswhen subjected to the conventional 1-T bend, 2-T bend, 8 inch-pounddirect impact, and 22 inch-pound reverse impact tests.

The following examples illustrate the best modes contemplated forcarrying out this invention.

EXAMPLE 1

An emulsion copolymer latex of styrene/isobutyl acrylate/hydroxyethylmethacrylate/methacrylic acid in the weight ratios of 38.5/50/10/1.5(50% by weight total solids in water) is blended with a water-solublemelamine-formaldehyde adduct modified by reaction with methanol, themole ratio of melamine/formaldehyde/methanol being 1:7:12 (80% totalsolids in an equal volume mixture of isopropanol and butanol) and 1.3%based on the weight of total solids of a polyethylene glycol having amolecular weight of about 20,000 (Carbowax 20M). The weight ratio of thecopolymer to the adduct in the mixture of 60:40. The pH of thecomposition is adjusted to 9.5 by the addition of triethylamine. Theresultant composition has a viscosity of betweeen 150 to 300 cps, atotal solids content of 55% by weight and a density of 8.9 pounds pergallon. The weight ratio of water to alcohol in the system is 88:12. Thecomposition is direct roll coated on 90 pound tin plated steel as aclear over-print varnish for can bodies at 150 feet per minute with aurethane roll of Durometer 20. The flow and leveling of the compositionis very good. The coated panels are baked at 325° F. for 10 minutes toobtain a uniformly smooth, hard coating having good mar resistance.

EXAMPLE 2

An emulsion copolymer latex of styrene/butylacrylate/methacrylamide/methacrylic acid in the weight ratios of42/54/2.5/1.5 (50% by weight total solids in water) is blended with awater-soluble urea-formaldehyde adduct modified by reaction withmethanol, the mole ratio of urea/formaldehyde/methanol being 1:2.5:3(80% solids by weight in isopropanol). The weight ratio of the copolymerto the adduct is 60:40. To 80 parts by weight of this composition isadded 20 parts by weight of a melamine-formaldehyde adduct modified byreaction with methanol, the mole ratio of melamine/formaldehyde/methanolbeing 1:7:12, and 1.3% by weight of a polyethylene glycol having amolecular weight of about 20,000 (Carbowax 20M). The pH of thecomposition is adjusted to 9.7 by the addition of triethylamine. Thecomposition has a total solids content of 60% by weight and a viscosityof 50 cps. A ball mill grind is prepared by ball-milling 225 parts byweight of this composition, 225 parts by weight of DuPont Rutile R-960pigment, 10 parts by weight of solvent (water/butyl Cellosolve in theratio of 1:1), and small amounts of a pigment dispersant, wetting agentand defoamer. The mixture is ball-milled for 16 hours and the pigmentedcoating is applied by a directroll coater having urethane rolls of highDurameter rubber (20 Durometer) onto 90 pounds tin plated steel as abase white coat for can bodies at 150 feet per minute. Th flow andleveling characteristics of the coating composition are very good. Thecoated panels are baked for 10 minutes at 325° F. Uniformly white,smooth coatings are thus obtained.

EXAMPLES 3 and 4

The process of Example 2 is repeated substituting for Carbowax 20M anequivalent amount of Carbowax 400 (Example 3) and Carbowax 600 Example4). The compositions exhibit good flow and leveling characteristics whendirect roll coated on a metal substrate.

When each of the above examples, 1 to 4, is repeated omitting theCarbowax, the composition does not exhibit acceptable flow and levelingcharacteristics.

EXAMPLE 5

The process of Example 2 is repeated substituting a poly(ethylene oxide)(3% by weight based on the solids) having an average molecular weight ofabout 100,000 (Polyox WSRN-10) for the polyethylene glycol of thatexample. The composition has improved flow and leveling characteristicsover that obtained when the poly(ethylene oxide) is omitted.

EXAMPLE 6

A blend of an emulsion copolymer latex and a ureaformaldehyde adduct isprepared as described in Example 2 and 2% by weight of the liquid blendof triethylamine and 2.5% by weight of Carbowax 20M are added. Thecomposition has a total solids content of 60% and a pH of 9.0

Ball mill grinds are prepared from the following formulation, partsbeing by weight: 45 parts DuPont rutile R-960 pigment, 45 parts of theblend described above, 10 parts solvent (water/butyl Cellosolve in theratio of 1/1), 0.75% pigment dispersant, 0.5% wetting agent, 0.5%defoamer. The mixture is ball-milled for 48 hours. This gives a solidscontent of about 54%.

For the clear coating, the blend of the urea-formaldehyde adduct andlatex is simply diluted to 45% total solids with the same solvent, and1% by weight of a wetting agent is included in the composition. In thisand the following examples, the compositions are direct roller coated(Union Tool No. 5) at 150 feet per minute with hard rollers of neoprenerubber or polyurethane, to give a dry-film thickness of 0.2 to 0.4 milson both the dried coatings.

The pigmented coating is applied to 90 pound tin plated steel and toanodized aluminum 0.025 inch sheet in thickness, and panels are bakedfor 10 minutes at 325° F., a wet ink is applied, and the clear overprintvarnish immediately applied over the wet ink and baked at 325° F. for 10minutes. The flow and leveling characteristics of both the clear and thepigmented coatings are acceptable.

EXAMPLE 7

Example 6 is repeated but with a latex prepared by emulsionpolymerization of styrene/butyl acrylate/hydroxyethylmethacrylate/methacrylic acid in the ratio of 43/54/1.5/1.5. The flowand leveling characteristics are good.

EXAMPLE 8

Example 6 is repeated with a latex polymer prepared by emulsion methodscontaining styrene/butyl acrylate/methacrylic acid in the ratio of44.5/54/1.5. It will be noted that this polymer is outside of theinvention.

The coatings of Examples 6, 7, and 8 are then tested for gloss, forhot-stacking to determine whether cured coatings would stick to oneanother or to bare metal, tested for fabrication by forming into screwcaps for bottles with a baking schedule including rebaking the panelsfor 10 minutes at 400° F. and for 30 minutes at 400° F. The caps formedfrom the panels are then subjected to dry heat for 10 minutes at 400° F.Another batch of the caps prepared from the panels are subjected toheating for 1 hour with steam under pressure with the gauge pressurebeing 15 pounds and the temperature 250° F. The gloss of the base whitecoat of Examples 6, 7, and 8 is comparable at an angle of 60° in thegloss reading device. At 20°, the gloss of Examples 7 and 8 is slightlybetter than that of Example 6. The hot-stacking test shows Example 6 tobe slightly superior to Example 7, Example 8 being markedly inferior tothe products of the other examples. After the clear coating is appliedover the baked pigmented coating and baked for 10 minutes at 325° F.,the hot-stack results showed the product of Example 6 to be markedlysuperior to the other two, and that of Example 7 superior to that ofExample 8, which was unacceptable. After being subjected to overbakingor rebaking the panel of Example 8 at 400° F. for 30 minutes, capsformed therefrom appeared dull. The properties as far as top cracking,knurl failures, and defects in the body, base, lip, and ridge areas ofthe cap were generally comparable. Caps from the panel having received a30 minute overbake at 400° F. of Examples 6 and 7 after being subjectedto dry heat as described above, had a trace of cracking in the lip area.The product of Example 8 again failed the steam processing test, theproducts of Examples 6 and 7 being satisfactory after processing, insome cases with a few blisters or a slight wrinkling of the film, wherethe panel had been overbaked for 30 minutes. The caps of Example 8 weredull, rusted, and water spotted after processing. The remainder of thetests were comparable for the fabrication, dry heat, and processingtests.

In each of the foregoing examples, the cured coating was between about0.3 mil and 0.5 mil.

We claim:
 1. A pigmented or unpigmented thermosettable compositionadapted for direct-roller coating of metals comprising an alkalineaqueous blend having a binder consisting essentially of:A. a latex of awater-insoluble addition copolymer (2) 18 to 68% of at least one esterof acrylic or methacrylic acid, (3) at least one olefinicallyunsaturated monomer having at least one of an amide and a hydroxylgroup, and (4) on olefinically unsaturated monomer having a carboxylgroup, the amount of (3) being from 1.5 to 10% when said monomercontains an amide group and from 1.5 to 15% when said monomer contains ahydroxyl group, and the amount of (4) being from 0.5 to 5% based on thetotal weight of monomers, the Tg of the polymer being below 45° C., thetotal of (1), (2), (3), and (4) being 100, and B. a water-solublecondensation products of urea or a triazine with formaldehyde or awater-soluble methanol or ethanol ether thereof,the weight ratio of A toB on a solids basis being from 35:65 to 65:35, the minimum film-formingtemperature of the composition being no higher than about 25° C., thesolids content of the composition being between 40 and 80% by weight,said composition containing from 1 to 5% by weight based on the weightof the solids in the composition of a water-soluble rheology modifier inthe form of a polyethylene glycol.
 2. The composition of claim 1 inwhich B is a urea-formaldehyde reaction product or amelamine-formaldehyde reaction product, or a mixture thereof.
 3. Thecomposition of claim 1 in which B is the reaction product of aurea-formaldehyde adduct with methanol, the mole ratio ofurea:formaldehyde:methanol being in the range of 1/1.75-3/2-3.5; or amelamine-formaldehyde adduct with methanol, the mole ratio ofmelamine:formaldehyde:methanol being in the range of 1/7-10/12-17, or amixture thereof.
 4. The composition of claim 3 in which (1) is styreneor vinyltoluene, (2) is an ester of at least one of acrylic acid and analkanol having from about 2 to 12 carbon atoms, or an ester ofmethacrylic acid and an alkanol having from about 6 to 14 carbon atoms,(3) is at least one of methacrylamide, acrylamide, hydroxyethyl acrylateor methacrylate, or hydroxypropyl acrylate or methacrylate, and (4) isacrylic acid, methacrylic acid, or itaconic acid.
 5. The composition ofclaim 4 in which (1) is styrene, (2) is butyl acrylate, (3) is at leastone of hydroxyethyl methacrylate and methacrylamide, and (4) ismethacrylic acid, the respective relative amounts by weight of (1), (2),(3), and (4) are in the ranges up to 28-52/40-68/1.5-5/.5-2, the totalbeing 100, and which composition contains a volatile tertiary amine. 6.The composition of claim 5 in which the rheology modifier is apolyethylene glycol having a molecular weight of from about 400 to100,000.
 7. The composition of claim 5 containing from 10 to 20% byweight of the solids in the composition of a liquid organic coalescentfor the uncured polymer mixture.
 8. The composition of claim 5containing a pigment, the relative weight of pigment to A plus B beingfrom 5:95 to 60:40, the total being
 100. 9. The composition of claim 5in which the ratios of monomers 1/2/3/4 and 35-50/45-60/1.5-3/.5-2.